1. Field of the Invention
The present invention relates to a PN code generating apparatus applicable to a mobile device in a mobile communication system in which an intermittent reception is performed, for instance a CDMA (Code Division Multiple Access) communication system, and to a mobile radio communication system.
2. Description of the Related Art
Recently a mobile communication using a CDMA system has attracted public attention in a digital mobile communication field. In the U.S.A., the standardization of a mobile communication system in a CDMA system was performed by TIA (Telecommunications Industry Association) , which is summarized in "Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Digital Cellular System" (IS-95-A) and so on.
In a CDMA system, transmission data are specturm spread with a spreading code that is different each channel. For instance, in IS-95-A includes a short PN code of 15th order (the period is about 26 ms) and a long PN code of 42th order (the period is about 41 days) consumed In spectrum spreading. And the long PN code Is also used in the scramble for a forward link and in the assigning of an insert position of power control bit.
FIG.1 illustrates a schematic configuration of a conventional PN code generating apparatus. As an example, the case of PN code of 42th order, which requires 42 delay elements in a shift register, is illustrated. PN code generating section 100 comprises a feedback shift register composed of 41 EX-ORs (EXclusive-OR circuit) 101, 42 one-clock-delay elements 102, 42 primitive polynomial coefficients 103 and 42 multipliers 104.
In the PN code generating apparatus described above, the initial values of delay elements are set so that all values are not 0 at the same time, and the value of delay element 102 is shifted corresponding to an input of shift clock 105 considering the feedback of the value of the last slot.
Any output of delay element is obtained as a PN code.
In a CDMA mobile communication system, a mobile device sets the initial value of delay element 102 at a system timing in the process of the synchronization acquisition with a base station, then generates a PN code using a chip rate consumed in spreading in a CDMA system as a shift clock.
A mobile device in a mobile communication system performs the monitoring reception to check a call once in the specific period predetermined with a base station while waiting. This is called an intermittent reception, in which as many circuits as possible except a timer for measuring a timing for the next monitoring reception are turned off during the non-reception period so as to reduce the consumed electric power.
However in a conventional PN code generating apparatus, since it is necessary to keep the synchronization of a code pattern of a long PN code with much longer period than the intermittent reception period even during the non-reception period, it is not possible to turn off the apparatus, which brought the problem that the reduction of electric power can not be achieved.